Loran-C is a pulsed, low-frequency, (100 Khz) hyperbolic radio navigation system. Loran-C radio navigation system chains employ three or more synchronized ground stations that each transmit periodic radio frequency pulse trains having, at the respective start of transmissions, a fixed time relationship to each other. The first station to transmit is referred to as the master station, while the other stations are referred to as the secondary stations. The pulse trains are radiated to receiving equipment generally located on aircraft or ships whose positions are to be accurately determined. Each pulse of the pulse trains transmitted by each of the master and secondary stations has an extremely accurate envelope shape; each pulse train is transmitted at a constant, precise repetition rate called the Group Repetition Interval; and each pulse in a group is separated in time from a subsequent pulse by a precise, fixed time interval. In addition, the secondary station pulse train transmissions are delayed a sufficient amount of time after the master station pulse train transmissions to assure that the time of arrival at receiving equipment anywhere within the operational area of the particular Loran-C chain will follow receipt of the pulse trains from the master station.
Since the series of pulses transmitted by master and secondary stations is in the form of pulses of electromagnetic energy which are propagated at a constant velocity, the difference in time of arrival of pulses for a master and a secondary station represents the difference in the length of the transmission paths from the transmitting stations to the Loran-C receiving equipment. The locus of all points on the Loran-C chart representing a constant difference in distance from a master and a secondary station, as indicated by a fixed time difference of arrival of the 100 Khz carrier pulse trains, is described by a hyperbola. The Loran-C navigation system makes it possible for a navigator to utilize this hyperbolic relationship for two or more pairs of stations to determine position using a Loran-C chart on which are located families of intersecting hyperbolic curves in a manner well known in the art. The modern-day Loran-C navigation system provides equipment position location accuracy within 200 feet with a repeatability of within 50 feet.
The discrete pulses radiated by each master and secondary Loran-C transmitter are characterized by an extremely precise spacing of 1000 microseconds between adjacent pulses. To insure such precise time accuracy, each master and secondary station transmitter is controlled by a cesium frequency standard clock and clocks of master and secondary stations are synchronized with each other.
To make precise time difference of signal arrival measurements a specific point on the waveform or the zero crossing of a specific carrier frequency cycle of each pulse must be located. The specific point or zero crossing is located and used to make the time difference of signal arrival measurements. In actual operation noise and spurious signals at and about the Loran-C carrier frequency makes the task very difficult.
The operation of the Loran-C radio navigation system is described in greater detail in a pamphlet put out by the Department of Transportation, United States Coast Guard, No. CG-462 dated August, 1974, entitled "Loran-C User Manual".
Thus, there exists a need in the prior art for apparatus and techniques used to accurately locate a specific point on each Loran-C pulse used to make accurate time difference of signal arrival measurements in the presence of noise and spurious signals which decrease the signal-to-noise ratio of the desired signal.